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Turing model: Nodal-Lefty system

Although it is claimed in the text that general reaction-diffusion system is unimportant in biology, the very basic Turing mechanism of pattern formation, that is, fast diffusing inhibitor + slow diffusing activator to make patterns, is utilized in the basic developmental process 

The following is a summary from Integrative Natural History Chapter 2:

 2.1.8 Turing model is used Nodal-Lefty system [1]

Analysis of Nodal and Lefty gradients revealed that Nodals have a shorter range than Lefty proteins. Pulse-labeling analysis indicated that Nodals and Leftys have similar clearance kinetics, whereas fluorescence recovery assays revealed that Leftys have a higher effective diffusion coefficient than Nodals. Turing model is supported.

 A less diffusive activator (Nodal) induces both its own production and that of a more diffusive inhibitor (Lefty). The Nodal/Lefty reaction-diffusion system is further constrained by pre-patterns and rapid cell fate specification; thus, the system results in graded pathway activation during mesendoderm induction and exclusive pathway activation on the left during left-right specification.

[1] M¨uller, et al. Differential Diffusivity of Nodal and Lefty Underlies a Reaction-Diffusion Patterning System S 336 721 (2012).

Nodal/Lefty activator/inhibitor system during zebrafish embryogenesis.

[2] 〈 Lefty-Nodal system 〉 Nodals activate signaling during mesendoderm induction and left-right patterning, whereas Leftys block pathway activation. The Nodal/Lefty system fulfills two of the tenets of activator/inhibitor reaction-diffusion models: (i) Nodals are short- to mid-range activators that enhance their own expression, and (ii) Leftys are long-range inhibitors that are activated by Nodals.

Lefty (Wikipedia): Lefty (left-right determination factors) are proteins that are closely related members of the TGF-beta family of growth factors. These proteins are secreted and play a role in left-right asymmetry determination of organ systems during development.

Lefty proteins function as an antagonist of the Nodal Signaling pathway. Nodal is another signaling protein which is responsible for gastrulation, left-right patterning and induction of the primitive node. As NODAL protein diffuse through an embryo, it triggers Nodal Signaling within tissues with the required receptors and coreceptors. Activated nodal signaling leads to the transcription of the lefty gene. The protein is then expressed, proteolytically cleaved, and finally secreted. Secreted lefty binds to EGC-CFC proteins like one-eyed pinhead in zebrafish keeping the essential cofactor from associating with NODAL/Activin-like receptor complex. This will effectually block Nodal Signaling.